Frequency diversity is one of the characteristics that should be considered in designing wireless communication systems, especially for ones that operate over a wide frequency band such as WiMAX and 3GPP LTE. In addition to spatial and temporal diversities, signals transmitted over a wide frequency band experience independent fluctuations across frequencies. This phenomenon is generally called “frequency selective fading.” Frequency diversity is ignored in conventional WiFi systems because these systems use a channel as a whole. However, adoption of OFDM in 802.11 WLANs triggered recent research interests in harvesting gains from frequency diversity. The importance of frequency diversity research becomes more important than ever as IEEE 802.11 working group (WG) is standardizing the use of wider channels. For example, 802.11n can already use a 40 MHz channel by Phased Coexistence Operation (PCO) and 802.11ac will provide up to a 160 MHz channel. Accordingly, several Wi-Fi protocols exploiting frequency diversity have already been proposed recently in academia.
To harness frequency diversity, a wireless communication system must provide channel quality estimation functionality.
Acquiring channel quality information consumes time and frequency resource that ideally should be used for data transfer. For example, many current wireless systems estimate channel quality using a training sequence (pilot) in a preamble or spend dedicated time only for the channel estimation purpose. Moreover, for N by N Multiple-Input-Multiple-Output (MIMO) systems, N2 channels have to be estimated resulting in substantial protocol inefficiency. In this case, the high data throughput of a MIMO system cannot be achieved due to the large overhead of channel estimation. In short, there is a trade-off between frequency diversity gains and protocol efficiency.
The research approaches to achieve frequency diversity gains are categorized into two groups; (i) variants of Wi-Fi systems that improve the protocol efficiency and (ii) frequency diversity aware protocols for various wireless networks such as WiMAX, 3GPP LTE, and Wi-Fi networks. However, none of them explore both of the conflicting objectives—i.e., reduction of channel estimation overhead and protocol efficiency—simultaneously. Most previous work emphasizes mainly one side of these since the two objectives are considered as orthogonal to each other (but it is not true as we have discussed above). Also, frequency diversity aware studies are highly theoretical rather than practical, i.e., these researchers solved the channel allocation problem assuming the perfect channel information is given.